Glass and other transparent materials can be coated with transparent semi-conductor films such as tin oxide in order to reflect infra-red radiation. Such materials are useful in providing windows with enhanced insulating value (lower heat transport) for use in architectural windows, etc.; see for example, RE 31,708. Coatings on glass of tin oxide in combination with other coatings, such as iridescence--suppression coatings, are now enjoying commercial acceptance.
Another important glass coating is titanium nitride, which has optical properties that make it very efficient in controlling undesired solar heat gain through windows of buildings in warm climates.
When a glass surface is coated with tin oxide, a coater deposits the tin oxide on a moving glass surface. Ideally, it would be desirable to control the fluid flow characteristics of the reactants which form the tin oxide and the spatial relationship between the coater surface overlying the moving glass surface, such that the tin oxide which is formed, would only deposit on the moving glass surface. As a practical matter this has not been possible to achieve with the result that the tin oxide also coats the coater surface overlying the glass surface on which the tin oxide is deposited. When the tin oxide is formed by reaction of stannic chloride vapor with water vapor, a hard glossy deposit of tin oxide forms on the coater surface, which can be made of graphite or other corrosion-resistant materials such as nickel-based metal alloys (e.g. Inconel (trademark of Huntington Alloys, Inc.) or Hastelloy (trademark of Haynes International, Inc.)).
A similar effect occurs when glass is coated with titanium nitride by the reaction of titanium tetrachloride with ammonia, according to U.S. Pat. No. 4,535,000. A hard layer of titanium nitride forms on the coater, as well as on the glass.
After a production run, the coater surface must be cleaned before it is used again. Generally the tin oxide is removed by scraping. This procedure suffers from certain disadvantages. The contour of the graphite or metal is distorted because it is softer than the tin oxide and areas free of tin oxide are scraped more than areas where the tin oxide is attached. Patches of adherent tin oxide remain on the surface and an uneven surface still results. A similar problem is found with the removal of titanium nitride from its coating apparatus.
It is known to use zinc powder and hydrochloric acid to etch tin oxide. However, this method is not convenient for thick layers of tin oxide, say ranging between 0.5 to 2.0 mm thick nor is it easily practiced over large areas, say for example 3 m.sup.2. Similarly, titanium nitride cannot be dissolved in any solvents or acids. Titanium nitride does dissolve slowly in boiling mixtures of concentrated hydrochloric and nitric acid (aqua regia), but such a treatment of a large coater would be impractically dangerous to workers and destructive of the coater.